1. Field of the Invention
This invention relates generally to circuits for use in recording and playback system and, more particularly, to semiconductor amplifier circuits designed for use in magnetic tape recording and playback systems.
2. Description of the Prior Art
In the past, it was generally necessary among other things to change from the recording to the playback mode and vice versa by interchanging the input and output terminals of the amplifier. Typically, a six pole double throw switch was required to change;
(a) The input to the preamplifier. PA0 (b) Preamplifier equalization. PA0 (c) The input to the volume control for setting the loud speaker level to a predetermined level to the recording head, PA0 (d) The Automatic Audio Level Control (ALC) (on or off), PA0 (e) The output drive (to the loud speaker amplifier) switched to the recorder head, PA0 (f) The bias oscillator (on or off).
This complicated switching mechanism of prior art record/playback systems has created mechanical and electrical problems.
The voltage gain of an amplifier designed for tape recording and playback is generally between 1000 and 100,000 times. Because of this high gain, there is a necessity for isolating the input and output circuitry of a high gain amplifier to avoid feedback or regeneration which has always presented a design difficulty and was a potential source of electrical or mechanical failure in tape recorders.
As the state of the art in tape recorder system design advanced through the usage of transistors and other miniature components, a point was reached where the prior art playback/record switch mechanism became larger than the amplifier electronics and thus represented a substantial part of the complexity and cost of the recorder system.
It was recognized quite early that the stability of record/playback switching could be improved by grounding the movable elements of the transfer switch (see U.S. Pat. Nos. 2,971,063 to Genning, issued Feb. 7, 1961 and 3,360,615 to Knockenhauer et al. issued Dec. 26, 1967). Electronic switching has also been previously suggested as a desirable alternative to mechanical and electro-mechanical switching arrangements for use in playback/record (P/R) systems (see U.S. Pat. No. 2,853,559 to Leonard, issued Sept. 23, 1958.). However, this prior art patent used a balanced bridge and bulky iron cored components which were costly and sensitive to hum pickup.
Thus, a need existed to provide a single semiconductor chip that would have a simple, reliable P/R switching feature plus other important features such as, for example, a wide range automatic controller of the recording level and an improved metering circuit which assists the operator in preventing tape overload. Voltage regulation circuits to isolate the active electronic circuit portions from the effects of supply voltage and temperature changes were also needed.